Quantum Cascade (QC) lasers are ideal mid-infrared semiconductor light sources for molecular detection in applications such as environmental sensing or medical diagnostics. For such applications, researchers have been making great efforts to improve the device performance. Recently, improvements in the wall-plug efficiency (WPE) have been pursued to realize compact, portable, power-efficient, and/or high-power QC laser systems. However, advances were largely incremental, and especially the basic quantum design had remained unchanged for many years, with the WPE yet to reach above 35%. A crucial factor in QC laser performance is the efficient transport of electrons from the injector ground level to the upper laser level in the laser active regions. The stronger the coupling between these two levels, the faster electrons can be transferred into the active region and the better the performance of the device. This transport process was described as limited by the interface-roughness-induced detuning of resonant tunneling in Khurgin, J. B. et al. Role of interface roughness in the transport and lasing characteristics of quantum-cascade lasers, Appl. Phys. Lett. 94, 091101 (2009), but this limitation has not been addressed in actual QC lasers.
Quantum Cascade lasers are based on intersubband transitions in semiconductor quantum wells. Photons are generated when electrons transported into the active regions from the preceding injector regions undergo radiative transitions between the upper and lower laser levels and are subsequently extracted into the next injector regions. The electron transport from the injector region to the downstream active region occurs via resonant tunneling between the injector ground level and the upper laser level. The tunneling rate, as well as many other performance related parameters, can be engineered through quantum design, e.g., the design of the coupling strength, which is defined as half of the energy splitting between the injector ground level and the upper laser level when they are in full resonance. Theoretical analyses show that a fast tunneling rate is a critical factor for achieving high laser WPE. On the one hand, the faster the tunneling rate, the higher the maximum operating current density that can be supported, and therefore the higher the current efficiency, i.e., how far above threshold the laser is operated, which is an important factor of the WPE. On the other hand, the internal efficiency also benefits from a faster tunneling rate, because it reduces the electron population in the injector region, and thus minimizes the leakage current from the injector ground level to the lower laser level or the continuum energy levels.
With practical growth techniques, interfaces between adjacent semiconductor layers are not perfectly smooth, but in fact are rather rough on the scale of atomic layer steps of a few Å, which is significant compared to the typical semiconductor layer thicknesses of ˜10-50 Å in QC lasers. Furthermore, in the InGaAs/AlInAs/InP material system different interfaces have generally unrelated roughness because the width fluctuations of adjacent barriers and wells are unrelated. As a result, such interface roughness introduces significant detuning to the energy levels in resonance, which plays a crucial role in reducing the tunneling rate between the injector and active regions and thus the laser WPE. This effect has been greatly underestimated until recently, so that conventional designs failed to incorporate an adequate mechanism to reduce its negative influence. This effect has been recently re-evaluated and its importance modeled for laser gain. Theoretical calculations show that the interface-roughness-induced detuning to the resonant tunneling is in fact much larger than the broadening of the radiative transition which had previously been used as the limiting factor for the gain. With the coupling strength in conventional designs, the achieved gain is a factor of 2 to 3 lower than the maximum achievable value. Consequently, there is a need in the art for QC lasers that overcome the loss in gain due to interface-roughness-induced detuning
Apart from roughness-induced inefficiencies, broadly tunable QC lasers are ideal candidates for multi-analyte mid-infrared spectroscopy applications; however, the maximum tuning range is usually limited by the inherently narrow linewidth of intersubband transitions. Several techniques have so far been demonstrated to achieve a broad gain spectrum of QC lasers. Heterogeneous cascade designs comprise several sub-stacks optimized for different emission wavelengths; “bound-to-continuum” QC lasers have more than one lower laser state, therefore, generating multiple transitions in the same active region. Both methods achieve broad gain spectra by overlapping multiple optical transitions. Actually, even conventional QC laser designs based on resonant tunnelling from the injector ground state to the upper laser state intrinsically have two optical transitions at slightly different wavelengths. At resonance, the injector state and the upper laser state form a doublet from where two optical transitions to the lower laser state happen in parallel. In traditional designs, the energy splitting at resonance is usually about 5 meV for optimized peak gain and transport properties. Nonetheless, despite these approaches a need remains for broadband QC lasers with increased gain.